A window air conditioner fits a window in a way furniture never does: it has to sit in the opening, fill it side to side, and lock under a raised sash. Get one measurement wrong and the unit either will not seat or leaves a gap that leaks air all summer. Before you buy, you need the opening width between the tracks, how far the lower sash actually raises, the sill depth, and the window type, because a casement or slider needs a different unit entirely than a double-hung. This guide walks each measurement and the support and power checks that go with them.
Window air conditioners are built for the window they sit in. A standard unit needs a double-hung or single-hung window, where a lower sash lifts up and the unit fills the width with side panels (accordion curtains). A horizontal slider or a casement (crank-out) window cannot take a standard unit, because there is no sash to drop onto the top of the unit. Those windows need a vertical (tower-style) slider AC or a through-wall unit. Identify the window type before you measure, because it decides which dimension is the limiting one.
| Unit class (cooling) | Min window width | Max window width | Minimum sash opening height |
|---|---|---|---|
| Small (5,000 to 6,000 BTU) | 23 in | 36 in | 13 in |
| Medium (8,000 to 12,000 BTU) | 23 to 27 in | 36 to 39 in | 13 to 16 in |
| Large (14,000 to 18,000 BTU) | 27 to 29 in | 39 to 42 in | 16 to 20 in |
| Vertical slider / casement unit | 15.5 in (width) | 24 in (width) | 20 in (sash height varies) |
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Four numbers decide nearly every fit check. Get these right and the rest follows.
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Frequently asked
Measure the opening width between the left and right tracks (this is the window width the unit specs quote), the height the lower sash raises from the sill, and the interior sill depth. Confirm the window is double-hung or single-hung. Compare the opening width to the unit minimum and maximum window width, and the unit body height to the sash opening.
01Most standard units fit double-hung windows 23 to 36 inches wide that raise at least 13 to 16 inches. Larger units (14,000 BTU and up) often need 27 to 42 inches of width and 16 to 20 inches of sash height. The unit listing gives an exact minimum and maximum window width, and those are hard limits.
02Not a standard unit. Casement (crank-out) and horizontal slider windows have no sash to drop onto the top of the unit, so they need a vertical tower-style slider AC or a through-wall unit. Identify the window type first, because it changes which unit you can buy and which dimension limits the fit.
03For anything above the ground floor or any unit over about 60 lbs, yes. The sill and raised sash alone are not rated for a heavy unit hanging out the window. A bracket bolted to the exterior wall carries the load and sets the slight downward tilt the unit needs to drain. Many buildings require one.
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